Tracking Issue for const conversion methods that don't require const traits

[clarfonthey]

Feature gate: #![feature(const_convert_methods)]

This is a tracking issue for various methods used in the constification of the Deref and DerefMut traits which could be stabilised before const traits are stabilised. These methods themselves do not depend on any traits in their APIs and can be implemented in a way that does not use unstable const traits.

Public API

Constness is added to the following methods:

• Box::into_boxed_slice (Box<T> -> Box<[T]>)
• Box::into_pin (Box<T> -> Pin<Box<T>>)
• CStr::as_bytes
• CStr::as_bytes_with_nul
• CString::as_c_str
• CString::into_bytes
• CString::into_bytes_with_nul
• CString::into_string
• IntoStringError::into_cstring
• IntoStringError::utf8_error
• <Box<OsStr>>::into_os_string
• OsString::as_os_str
• <Box<Path>>::into_path_buf
• PathBuf::as_path
• path::{Component, PrefixComponent}::as_os_str
• <Box<str>>::into_boxed_bytes
• <Box<str>>::into_string
• String::from_utf8
• String::from_utf8_unchecked
• FromUtf8Error::as_bytes
• FromUtf8Error::into_bytes
• FromUtf8Error::utf8_error
• Box<[T]>::into_vec

rust-lang/libs-team#624 also proposes adding the following methods which do not exist. These are currently private, but could be made public:

• OsString::as_mut_os_str
• PathBuf::as_mut_path

Additionally, that ACP also proposes adding CString::as_mut_c_str, which is not implemented at all.

Explicitly Excluded APIs

While conversions from boxes to other collections are supported, because the reverse direction involves removing excess capacity, those conversions are omitted.

Although CString currently uses Box<[u8]> internally, this is an implementation detail that isn't relied upon. However, it also does prevent conversions from Vec for similar reasons.

Explicitly because the nul byte on CStrings can be popped off without deallocating, CString::into_bytes is made const.

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation: Constify conversion traits #144289
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Should CString be given a DerefMut impl?
• Should PrefixComponent be given AsRef<OsStr> and AsRef<Path> impls? (these are indirectly used for Component's implementation, but it doesn't have them itself)

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩